Climate scientists who have been warning of the dangerous effects of global warming now have the World Bank on their side, after a new report from that organization calling for action to prevent climate catastrophe.

"The World Bank did a great service to society by issuing this report," said Michael Mann, a climate researcher at Pennsylvania State University and the author of "The Hockey Stick and the Climate Wars" (Columbia University Press, 2012).

Climate deniers often claim that solutions to global warming are part of a "global socialist agenda," Mann told LiveScience.

"The fact that the World Bank — an entity committed to free market capitalism — has weighed in on the threat of climate change and the urgency of acting to combat it, puts the nail in the coffin of that claim," he said.

A changing world

The report, issued by the Potsdam Institute for Climate Impact Research and Climate Analytics for the World Bank, urges nations to work to prevent the Earth from warming 7.2 degrees Fahrenheit (4 degrees Celsius) past preindustrial averages. Already, global mean temperatures are running about 1.3 degrees F (0.8 degrees C) hotter than before the onset of the industrial revolution.

Likewise, carbon-dioxide concentration in the atmosphere is high and rising. As of September, the concentration was 391 parts per million, a record high, up from a preindustrial 278. That number is now rising by about 1.8 parts per million each year.

But what the World Bank warns of is an even bleaker future. Even if the world's nations deliver on their promises of emission limits and global warming mitigation, there is a 20 percent chance that the world will hit the 4 degrees C mark by 2100, according to the report. If emissions continue as is, the planet may reach that point by the 2060s.

International negotiators have agreed that warming should be limited to just half that, or 3.6 degrees F (2 degrees C), in that time. A world that is 2 degrees warmer would have its own consequences, but it is crucial to hold that line, the World Bank report argues. A 4-degree warming would mean a sea-level rise of 1.6 to 3.2 feet (0.5 to 1 meter) on average, with the tropics catching the brunt of the change.

Climate research also suggests tropical storms would strengthen and drought would increase across much of the tropical and subtropical world.

"A world in which warming reaches 4 degrees C above preindustrial levels (hereafter referred to as a 4 degree C world), would be one of unprecedented heat waves, severe drought, and major floods in many regions, with serious impacts on human systems, ecosystems, and associated services," the authors wrote in the World Bank report.

Climate scientists agree.

"I am inclined to think that things will break before we get there," Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, said of a 4-degree-C world. Ecosystems would change so much and agriculture would be so disrupted that the result would likely be "major strife, conflicts and loss of population," Trenberth told LiveScience.

Among the flashpoints, according to the World Bank report, would be sparse water availability, food insecurity and loss of resources such as coral reefs, which are threatened by acidification as more carbon dioxide is dissolved in the oceans. Coral reefs provide not only food to many local economies, but also tourism dollars. Areas becoming unsustainable would likely lead to mass exodus, creating environmental refugees, Mann said. [10 Surprising Results of Global Warming]

Avoiding the 4-degree world

Avoiding the 4-degree-warmer world is a matter of political will, said Mann, who sees signs of optimism, including increased awareness and more calls to transition away from fossil fuels.

"The alternative energies (wind, solar, geothermal, etc) are there," Mann wrote in an email to LiveScience. "We just need to deploy and scale them up by investing immediately in the necessary infrastructure."

Slowing the warming may be as useful as stopping it, Trenberth said.

"It is not just the absolute amount of warming, but also the rate at which
we change things to get there," he said. "Two degrees warming in 50 years is extremely stressful, but 2 degrees warming in 500 years is perhaps manageable through adaptation."

If the world fails to act, the world will become a more disrupted, damaged place, the World Bank concluded — and the poor will suffer most.

"The projected 4°C warming simply must not be allowed to occur — the heat must be turned down," the authors wrote. "Only early, cooperative, international actions can make that happen."